1. Field
The present disclosure relates to techniques for communicating optical signals. More specifically, the present disclosure relates to an optical waveguide that contains a vertical or a horizontal slot which includes an electro-optic material.
2. Related Art
Silicon photonics is a promising technology that can provide the large communication bandwidth and low power consumption needed to facilitate inter- and intra-chip interconnections. For example, a point-to-point communication network can be established using silicon-photonic devices and links to interconnect a large number of processor cores in a manner that achieves scalable performance with affordable manufacturing and energy costs. In addition, silicon photonics can be compatible with CMOS processing, which facilitates high-yield, low-cost integration with other devices and circuits.
One of the building blocks in inter- and intra-chip silicon-photonic interconnects is an optical modulator. An optical modulator converts an electrical data signal into an optical signal. Typically, an optical modulator provides: high speed (for example, in excess of 10 Gb/s), low power consumption, low optical loss, a high ON/OFF extinction ratio and compact size.
Many existing silicon-based optical modulators operate based on the free-carrier plasma-dispersion effect in silicon. In particular, the index of refraction of silicon decreases as the densities of electrons and holes (i.e., free carriers) increase. In order to use this effect for data modulation, carrier densities in an optical waveguide in a silicon-based optical modulator are typically electrically modulated, thereby changing the index of refraction and, thus, the phase of light propagating in the optical waveguide. This phase modulation is converted into an optical intensity modulation (i.e., ON/OFF switching) by including the phase-modulation optical waveguide in a Mach-Zehnder interferometer (MZI) or a ring resonator.
However, there are some problems with existing silicon-based optical modulators. In particular, the dependence of the index of refraction of silicon on the free-carrier density is weak. It is also associated with optical loss, and there is typically a relatively small overlap between the optical mode and a carrier-swept region in existing silicon-based optical modulators. As a consequence, when modulating the free-carrier density using carrier injection or carrier depletion, the size of a silicon-based optical modulator typically needs to be approximately one millimeter to generate a 180° phase shift in an MZI-type optical modulator.
By using forward-bias operation of a PIN diode to inject carriers, there can be a larger overlap region (relative to carrier-depletion operation) and, thus, a larger modulation of the index of refraction and the phase. However, the modulation speed is limited by the carrier diffusion velocity (which is on the order of a nanosecond). A modulation speed of 12.5 Gb/s has been achieved, but only by utilizing a pre-emphasized electrical signal (with Vpp equal to 8 V plus a 3.5 V pre-emphasized pulse), which significantly increased the power consumption. While carrier-depletion operation in a reverse-biased PIN diode is not affected by such a speed limit, it typically uses a higher voltage and/or a longer modulation length (for example, more than 2 V-cm) in order to obtain a 180° phase shift in an MZI-type optical modulator.
In addition, the power consumption associated with large MZI-type optical modulators often makes these components impractical in optical-interconnect applications (such as in high-performance computing systems). While a compact resonator (such as a micro-ring or a micro-disk that is smaller than hundreds of micrometers) with a small capacitance can be used, this type of optical modulator usually needs to be tuned in order to align its resonant wavelength with a carrier wavelength (such as a laser wavelength) in an optical interconnect because of the sensitivity to unwanted phase shifts associated with: environment temperature, fabrication variations, and modulation-bias condition and fluctuation. Furthermore, while carrier-injection tuning is efficient, it often introduces optical loss, which can significantly degrade the quality (Q) factor of a resonator. However, the alternative approach, thermal tuning, generally consumes too much power, and this problem is expected to become even more challenging as the critical dimensions of optical modulators are scaled to smaller values.
Hence, what is needed is an optical modulator without the above-described problems.